Pumping systems of many designs have been used to transfer material for application by spraying or other means. However, in the application of many coatings and other single and multi-component materials, special problems have been encountered. These problems include the need to precisely pump specified quantities of material and provide such materials at a near constant and continuous pressure at a remote location from the pump equipment. Further, the pump equipment must be capable of pumping large volumes of material at high pressures. In many cases, the materials being pumped are very caustic requiring the need for equipment which can fulfill these requirements even though it is subjected to continuous exposure to such materials.
In the past, various systems have been used. One widely used pump system for the application of multi-component polyurethane foams incorporates an air motor which drives dual proportion pumps, with each proportion pump designed to pump one of the components to be sprayed. The components are then mixed at a discharge gun prior to application. The air motor incorporates a piston moving in a cylinder with air being sequentially applied above and below the piston to reciprocate the piston. A reversing switch is actuated as the piston reaches a predetermined position in its travel and air pressure is alternated to the opposite side of the piston. Air on the driven side is then exhausted to the atmosphere. When the system is in the "off" position, air is normally maintained under pressure on one face of the air motor piston subjecting the seals to extremely large pressures during all times the unit is in use, even though in the "at rest" or "off" position.
The air motor drives one or more proportion pumps which pumps the fluid to the applicator gun. The proportion pumps each include a piston driven in a cylinder. The proportion pump is referred to as a double acting pump in that during one stroke the fluid loaded below the piston is in part pumped to the applicator gun. However, a portion of this material, and in most cases one-half of the volume, is pumped above the piston and around the piston rod, such material being discharged on the reciprocated stroke. These proportion pumps are designed in the manner in an effort to achieve the pumping of an almost equal amount of material on the downstroke as on the upstroke.
While these systems have met with some success, they have also presented substantial problems. First, to maintain pressure in the lines between the proportion pumps and the discharge gun, the air motor is constantly subjected to pressure, placing the seals and other components under continuous stress. Further, the proportion pumps are not efficient in that on one stroke, fluid being pumped is pumped into the cylinder above the piston and around the piston rod. Up to one-half of the pump volume is used simply to recharge and reposition the piston within the cylinder. However, during this stroke, line pressure is communicated to the area above the piston and because the pressure in the line is greater than the pressure in the cylinder, fluid previously pumped is forced backed into the cylinder above the piston and around the piston rod, resulting in the loss of volume and pressure in the entire system. Because this design permits previously pumped fluid to back-flow into the cylinder, requiring it to be repumped a second time, the system is highly inefficient.
Further, when the gun of the unit is shut off, with the air motor under continuous pressure, the piston in the proportion pump continues to creep in the cylinder causing fluid to move around the piston seals and applying loads to all of the components. This action causes substantial wear and premature failure of the seals. Likewise, the air motor is under continuous pressure during the operation of the system, even when spraying is not being accomplished.
Therefore, the presently used system does not provide an efficient design which will produce a high output at extended distances from the pump equipment while maintaining only a minimum pressure on the system components.